1. Field of the Invention
The present invention relates to optical communications networks and in particular to flight control systems and other systems for aircraft employing fibre-optic networks, i.e., so called fly-by-light systems. The invention is especially concerned with the provision of improved apparatus and methods for testing such fibre-optic networks.
2. Discussion of Prior Art
A fibre-optic communications network usually comprises at least one light source, a light modulator for modulating the light source according to variations in some parameter relating to one system component, information concerning which is to be transmitted as a light signal to one or more remote system components, fibre-optic light guides inter-connecting said system components, light demodulators for detecting and demodulating the received light signals and, transducers for converting the light signals to electrical or mechanical signals for activating the remote system components accordingly.
The points in the system at which light is coupled into or out of the fibre-optic light guides are called Nodes. Light from a single light guide may be split into a number of separate light guides simultaneously by means of devices known as star couplers.
For example, a flight control computer may be connected to several flying control actuators, e.g., ailerons, rudder, elevators, flaps, slats, engine throttle by means of a network of fibre-optic light guides. In order to provide emergency back-up arrangements, operational in the case of failure or partial failure of the main flight control system the computer the actuators and their receiving elements may be duplicated, triplicated or even quadrupled to form independent systems. The paths of the light guides forming the networks associated with those independent systems may also be varied to reduce the possibility of simultaneous loss of the main system and all its back-ups. The signals provided by the various elements of each independent system may be compared at various stages of their networks and subjected to logic voting arrangements to determine the most likely signal required to be acted upon of those presented or to determine the location of a failed link in the network and to make alternative routing arrangements.
Such systems are obviously complex, but it is vital to check as far as possible, both during manufacture and in routine servicing of operational aircraft, that the fibre-optic links between nodes and star couplers are sound. Such network checking procedures are known as Network Characterisation (or Network Loss Measurements).
The measurements are required to demonstrate the quality of the optical networks both during assembly and before the installation and connection of the Avionic equipment.
A typical known network comprises two 16 node star couplers and associated optical cabling and connectors. When assembled these networks can take several days to characterise using simple test equipment under laboratory conditions. This can be done by connecting an optical power meter at one node and then moving the transmitter around the network until all free ends had been checked. Having done this the power meter is moved on to the next node and the process repeated.
This task is onerous within the context of mass production of fighter aircraft. For example a 32 node network tested with light propagated in both directions would require 992 separate measurements to be taken and logged. Streamlining the network characterisation process within the production environment is therefore highly desirable and should result in substantial recurring cost benefits.